One way would be to get pipe the o/p and filter out the last line and get the no of deleted messages from there & assign it to a variable.

no_of_msgs=$(mailq | grep -v "^[^0-9A-Z]+" | grep MAILER-DAEMON | awk '{print $1}' | postsuper -d - | tail -1 | awk '{print $3}')

dt_time=$(date +'%Y%m%d %H:%M')

echo "$dt_time - ${no_of_msgs}" >>cron_purges